linux-user: do_setsockopt: fix SOL_ALG.ALG_SET_KEY

linux-user: do_setsockopt: fix SOL_ALG.ALG_SET_KEY

This setsockopt accepts zero-lengh optlen (current qemu implementation
does not allow this).  Also, there's no need to make a copy of the key,
it is enough to use lock_user() (which accepts zero length already).

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2197
Fixes: f31dddd2fc "linux-user: Add support for setsockopt() option SOL_ALG"
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20240331100737.2724186-2-mjt@tls.msk.ru>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 04f6fb897a5aeb3e356a7b889869c9962f9c16c7)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>